However, the salinity of the water is 

 not entirely determinative, either in the 

 case of eggs or fry. Jacobsen and Johansen 

 (1908) showed that cod eggs can change their 

 specific gravity in a day or less to corre- 

 spond to a salinity change of as much as 

 6°/oo, and Dahl (1909) points out that the 

 position of the fry in the water may be 

 varied by currents or even by the activity 

 of the fish themselves. 



There are many isolated observations 

 on buoyancy and on the distribution of eggs 

 and larvae in the ocean. Henly (1952) 

 reports that cod larvae are buoyant in 

 about 27.39°/oo, agreeing with Dahl's and 

 A. Dannevig's work. Within and beyond the 

 range of the table, eggs have been kept in 

 23.06°/oo to 27°/oo (Johansen and Krogh 

 (1914), 31.860/oo (Ryder 1886), 32.31°/oo 

 (Bonnet 1939), and 31.11°/oo to 32.36°/oo 

 (Carswell 1889). Hjort and Petersen (1905) 

 found pelagic young cod in about 35°/oo. 

 Larvae have been found in intermediate 

 layers in the Baltic from 35 to 75 meters 

 in 7.5°/oo to 14.2°/oo (Poulsen 1931), 

 probably because of the abundance of food 

 in these layers, although according to 

 table 2 this salinity is dangerously low. 



The net impression from the above is 

 that the question of suitable and optimum 

 ranges of salinity for eggs and larvae has 

 not been settled. Laboratory experiments 

 do not entirely confirm field observations, 

 and there is a possible difference between 

 American and European cod. 



As A. Dannevig and Sivertsen (1933) 

 point out, there may be a synergy between 

 salinity and temperature. There is, of 

 course, a physical relationship — sea water 

 of a given salinity will decrease in den- 

 sity as temperature increases above 0°. 

 But the effect on buoyancy is minor through 

 the temperature and salinity ranges in 

 which eggs and larvae are found. Below 10° 

 and 35°/oo no temperature change can cause 

 more than 0.001 change from the density at 

 0° (Knudsen 1901). 



At the relatively shallow depths in 

 which cod eggs and fry are found, the 

 effects of pressure on density are so mi- 

 nute as to be negligible (Sverdrup, Johnson 

 and Fleming 1942). 



Miscellaneous 



In addition to the effects of tem- 



perature and salinity on eggs and larvae, 

 other factors have been noted and in some 

 cases measured. 



Water movements which may carry the 

 eggs or young to areas not suitable for 

 them can be important, as discussed by John- 

 stone (1906) and Fish (1929). Sund (1924) 

 found that years in which there was less 

 snow in Norway were years producing good 

 year classes of cod. He postulates that 

 the runoff in spring from heavy snows dur- 

 ing the winter drives the fry-carrying 

 bank water offshore to areas not suitable 

 for the tiny fish. 



Fish (1929) considered that storms 

 might cause extensive damage to or destruc- 

 tion of eggs and larvae, and experiments 

 by Rollefsen (1930) showed that cod eggs 

 are indeed extremely susceptible to mechan- 

 ical shock such as might be caused by 

 storms or even surface waves caused by 

 strong winds. 



Poulsen (1931) found that cod larvae 

 avoid water with an oxygen content of less 

 than 4.9 cm. ■> per liter, much higher than 

 Sundnes' (1957) critical value of 2.7 ml. 

 per liter for mature fish. 



A. Dannevig and Sivertsen (1933), in 

 laboratory experiments, demonstrated that 

 a pH of 4.1 was fatal to cod larvae, which 

 survived in a range of pH 5.7 to pH 8.5. 



Henly (1952) observes that after a 

 certain stage of maturity the larvae must 

 leave the surface water for hydrostatic 

 balance, but that four meters depth was 

 sufficient in a hatchery pond. 



Schmidt (1931) concluded that some 

 unknown factor in inshore water other than 

 temperature and salinity has the effect of 

 decreasing the average number of vertebrae 

 in the cod. 



FEEDING AND GROWTH 



Temperature 



It has been observed that cod in 

 captivity grow about twice as much in July 

 and August as they do in March and April, 

 presumably due to the temperature differ- 

 ence (Anonymous 1935a). Dahl (1909) showed 

 that the growth of cod fry is associated 

 with temperature — that the fastest growth 



